This invention relates to an optical head for an optical disc, which reads out information recorded thereon by utilizing a light beam reflected by the disc. The invention further relates to an optical head or a magneto-optical disc, which also reads out information recorded thereon, by thermo-magnetically changing the magnetic polarity. The head can also be used as a recording head.
Conventionally, the prior art optical reader generally has a polarized beam splitter and a quarter wavelength panel (hereinafter referred to as QWP). Both, placed in an optical path between an optical disc and a polarized light emitter, function for detecting a light beam reflected by the disc, and for reflecting the reflected light beam in a predetermined direction to prevent the reflected light beam from returning to the emitter, thereby avoiding mis-functioning of the system. In the aforementioned optical reader, a polarized light beam from the emitter is changed into a linearly polarized light beam by the splitter, then to a circularly polarized light beam by the QWP to be irradiated onto the optical disc. The beam reflected by the disc reverses its rotational direction of polarization to pass through the QWP again by which the beam is changed into a linearly polarized light beam again. This beam has its plane of polarization 90.degree. different from that of the incident beam. The beam, then, is reflected by a polarized beam splitter in a predetermined direction to irradiate a light sensor. The prior art optical reader effectively detects a beam reflected by the optical disc because the reflected beam is prevented from returning to the emitter. But it has such a disadvantage in that the QWP is required to be placed on the optical path with its optical axis adjusted a strict 45.degree. to the P plane of polarization of the beam splitter. Such adjustment needs time and care, resulting in troublesome work in assembling the optical head for the reader. Conversely, an optical disc head according to the present invention requires no troublesome adjustment of its optical system, but allows simplified assembly while still effectively detecting a beam reflected by the optical disc.
Another prior art device for reading out information recorded on a magneto-optical disc utilizes the Kerr effect wherein a plane of polarization of a linearly polarized beam reflected by a magnetic substance is rotated to a certain angle depending upon the magnetic polarity and strength of the disc. This device is generally provided with a half mirror placed in the optical path between the magneto-optical disc and a light source, e.g., a laser diode or the like, which generates a linearly polarized light beam. The half mirror splits a light beam by a predetermined ratio (e.g. 50:50). When a light beam fed from the light source is reflected by the disc, the half mirror splits the reflected beam, and feeds the split beams to an information signal detection system and a servo signal detection system. Each system generates the information signal representing the information recorded on the disc, and a servo signal representing the irradiation condition on the disc, respectively. The intensity of a beam fed from the light source, however, weakens by a certain ratio each time it goes through the mirror. This phenomenon deteriorates the S/N ratio of the detected signal, and requires a light source of greater capacity. Since some portion of light reflected by the disc is not reflected by the half mirror, it returns to the light source, introducing noise into the system.